Bitumen or asphalt is the heaviest portion from the oil distillation process. Due to the different origins and distillations processes of such oils, the resulting bitumen may have a wide range of properties and characteristics. In the present invention, bitumen refers not only to the product from oil by direct distillation or from distillation of oil at reduced pressures, but as well to the products coming from the extraction of tar and bituminous sands, the products of oxidation and/or fluxation with carbonated solvents including paraffins and waxes of such bituminous materials, as well as blown or semi-blown bitumens, synthetic bitumens (such as those described for example in FR-A-2 853 647), tars, oil resins or indene-coumarone resins mixed with aromatic and/or paraffinic hydrocarbons and the mixtures of such, the mixtures of such bituminous materials with acids and the like.
The main application for bitumen is in asphalt mixtures where the bitumen is mixed with mineral aggregates that can be of different size, shape and chemical nature. These asphalt mixtures are particularly used for construction, repair and maintenance of sidewalks, roads, highways, parking lots or airport runaways and service roads and of any other rolling surfaces. In the present invention mineral aggregates are the product from quarries as well as aggregates recuperated from previous asphalt mixtures (Reclaimed Asphalt Pavement, RAP, as described for example in the AFNOR XP P98-135, December 2001, Asphalt Handbook, MS-4 7th edition, published by the Asphalt Institute, USA), products from building demolition and their mixtures. Other common components in asphalt mixtures are organic and inorganic fibers, such as glass, metal or carbon fibers, as well as cellulose, cotton, polypropylene, polyester, polyvinyl alcohol and polyamide fibers.
The bitumens, as defined above, are however sparingly used as such for the production of asphalt mixtures, mainly because of the unsatisfying mechanical properties of such mixtures. A number of various additives have therefore been developed since many years to reach satisfying asphalt mixtures that can withstand traffic wear and atmospheric conditions.
Polyphosphoric acid (PPA) is a commonly used bitumen modifier, which is capable of enhancing the mechanical properties of the bitumen. The effect of PPA on bitumen is well documented in the literature as described for example in FR-A-2 065 076, and in the article by J. F. Masson in “Energy and Fuels”, (2008), vol. 22, page 2637. PPA is able to modify the specification grade of the bitumen towards a harder one. Its use increases the ring and ball temperature, viscosity and reduces the penetration grade. As a consequence, it is useful to reduce the rutting of the road and increase the mechanical modulus of the asphalt mixture. As another consequence, it is necessary to increase the process temperatures for mixing the modified bitumen with the aggregates, storing, transporting, laying and compacting in comparison with asphalt mixtures employing neat bitumen.
In the present invention, polyphosphoric acid refers to all different existing grades and designations of polyphosphoric acids including pyrophosphoric acid, triphosphoric and metaphosphoric acids with 2 or more repeating units. Polyphosphoric acid also refers to the blends of two or more different polyphosphoric acids grades. Polyphosphoric acids according to the invention may further include one or more of phosphorous-containing moieties such as orthophosphoric acid, phosphoric anhydride (P2O5), phosphonic acid, pyrophosphates, and other similar compounds comprising at least one (OH—)P═O group.
Lately, there has been a lot of interest in reducing the production temperature of hot mix asphalts. Such interest in reducing the production temperatures of asphalt mixtures is not just driven by the energy savings and reduction of polluting emissions; it is also driven by the possibility to increase the asphalt mixture hauling time and the extension of the working season. Several solutions have been proposed, which consist in the addition of some kind of additives.
One of those solutions was exposed in U.S. Pat. No. 7,297,204 describing a method to produce an asphalt mixture at reduced temperatures by the addition of a water-in-oil dispersion. The water-in-oil dispersion includes one or several surfactants to allow for the dispersion of the water into the bitumen. Although this method provides a way to reduce the production temperature of an asphalt mix it would be incompatible with the polyphosphoric acid due to its great hygroscopic behavior. The contact of water with the polyphosphoric acid would break the chains of the acid into its elemental units, chains which are though responsible for it beneficial effects (J. F. Masson, ibid.). Further more, the PPA might be incompatible with many of the surfactant chemistry described in the text based on their basic nature.
In WO 2006/106222 and WO 2007/141458 different types of additives for the production of asphalt mixtures at reduced temperatures are described. The therein claimed additives do not lead to a change of the bitumen classification (penetration and ring and ball temperatures remain substantially unchanged). Actual field test are used as examples, showing that a reduction of about 40° C. up to 55° C. in the production and in the compaction of the asphalt mixture is possible, while achieving a good compaction level. The described additives are easy and simple to mix into the bitumen.
The procedures described in these documents also include waterless processes, which should be advantageous for the combined use with polyphosphoric acid. However, their combined use with polyphosphoric acid is not mention or described. The combined effect of such additives admixed with the highly reactive polyphosphoric acid cannot be easily predicted, in particular since the real mechanism of how the polyphosphoric acid enhances the bitumen properties is not fully understood. (J. F. Masson, ibid.).
Several advantages result from the reduction of production temperature, as described in many of the works cited above. These advantages include, among others:                Reduction of the fuel used in the production process.        Reduction of polluting emissions, related to the fuel consumed and the lower to temperature of the asphalt mixture.        Improving of the workers conditions during the laying off and compaction operations.        Better joint sealing between lanes posed at different times.        Decreasing of the bitumen oxidation during production of the asphalt mixture, extending the pavement life.        
In addition, the ability to lay off and compact an asphalt mix at lower temperatures can compensate for the supplementary cooling encountered when the hauling time is longer or/and when the weather conditions are cooler.
There is still a need for bitumen additives that can combine the beneficial effects and the advantageous properties of both:
the above mentioned additives for the production of asphalt mixtures at reduced temperatures, and
polyphosphoric acid for its final mechanical performances it confers to asphalt mixtures in wear conditions during the whole life of the rolling surfaces.